As it is known, in modern internal combustion engines, the high-pressure pump of the injection system is able to send fuel to a common rail having a predetermined accumulation volume of pressurized fuel, which feeds a plurality of injectors associated with the engine's cylinders. In general, the required pressure of the fuel in the accumulation volume for this type of system is defined by an electronic control unit, based on the engine's operating conditions.
Injection systems are known, in which a bypass solenoid valve, positioned on the pump's delivery line, is controlled by the control unit. When the engine runs at maximum speed but with reduced power, the flow rate of pump is excessive and the excess fuel is simply discharged by the bypass valve directly into the fuel tank. This bypass valve thus has the problem of dissipating part of the compression work of the high-pressure pump as heat.
Injection systems have been proposed in which the high-pressure pump has variable flow rate, so as to reduce the quantity of pumped fuel when the engine operates with reduced power. In one of these systems, the pump's intake line is fitted with a throttle solenoid valve for a restriction, which is controlled asynchronously by the control unit with respect to the operation of the pumping element, as a function of the pressure required in the common rail and/or the engine's operating conditions. The fuel taken in, downstream of the throttle solenoid valve and the restriction, has a very low pressure and, at low flow rates, makes little contribution to the force for opening the intake valves.
To this end, in known systems it is necessary to provide the usual return spring for each intake valve so as to guarantee opening even with minimal pressure downstream of the restriction. On one hand, this spring must be set in a very precise manner, whereby the pump becomes relatively expensive. On the other hand, the risk always remains that the intake valve is not able to open itself under the combined effect of the pressure exerted by the fuel on the intake valve and the depression caused by the pumping element in the relevant compression chamber, whereby the pump does not work properly and is easily subject to wear. In any case, if the pump has multiple pumping elements, it always gives rise to asymmetric delivery, especially under conditions of strong delivery choking.
In another known injection system, a throttle device has been proposed that comprises an on-off metering solenoid valve, which can be positioned on the intake line of the individual pumping element, or on an intake line common to the pumping elements. The metering solenoid valve has relatively high flow rate, so as to allow feeding the pumping element during a variable part of the intake stroke, of which the instant of the start and/or end of feeding is modulated, thereby the filling coefficient of the pumping elements is modulated.
If the control and actuation of this solenoid valve takes place synchronously with respect to the pump shaft's frequency of rotation (i.e. the metering solenoid valve is activated every revolution of the shaft, independently of the number of pumping elements that distinguish it), this throttle device has the drawback of having to synchronize and to time the operation of the metering solenoid valve with the position of the piston in each pumping element during the associated intake stroke. The same drawback is found if the activation frequency of the metering solenoid valve has a value equal to or a multiple of the intake stroke frequency of any pumping element (in particular, if the metering solenoid valve is synchronized with the intake stroke of the pumping elements; for example, for a pump with three pumping elements driven by a cam, its activation frequency is equal to three times the frequency with which the pump completes a revolution).
These systems, with flow regulated via an on-off metering solenoid valve on the intake line and controlled in a synchronous manner with respect to the rotational frequency of the pump and, in particular, systems in which the metering solenoid valve is controlled in a synchronous manner during the intake stroke of the pumping elements or with a multiple frequency of these strokes, present several other drawbacks that cause pressure oscillations in the common rail. First of all, it is necessary to distinguish between the causes that induce pressure oscillations over a relatively short time span, in the order of one engine cycle, and causes that induce pressure oscillations in the common rail over a time span in two or three orders of magnitude longer than the previous one. These two types of causes are additive and are substantially independent of each other.
Amongst the causes inducing pressure oscillations with a period equal to that of an engine cycle, the following should be mentioned:                irregular instantaneous flow rate of the high-pressure pump;        asymmetries in the volume of fuel delivered by the various pumping elements due to unequal setting of the intake springs;        injection events of the injectors and their timing with respect to the pump's delivery curve;        volume of the common rail; and        operating point of the engine.        
With regard to pressure oscillations with a period two to three orders of magnitude longer, the main cause is due to the small, or slow, timing variation, or slippage, of the instant of activation start of the metering solenoid valve, with respect to top dead centre of the reference pumping element.
In any case, the filling coefficient of the pumping elements mainly depends on the inevitable delay in the opening of the intake valve and is different from pumping element to pumping element as a result of the impossibility of evenly setting the intake valve springs, whereby the pumping elements work in a mutually asymmetric manner on each engine cycle.
Furthermore, especially in cases where flow choking is more extreme, the filling coefficient of a given pumping element is strongly influenced:                by the timing of the instant of activation or opening start, of the metering solenoid valve, with respect to the top dead centre of the same pumping element, and therefore by the depression downstream of the metering solenoid valve;        by the passage section of the metering solenoid valve;        by the interaction of activation of the metering solenoid valve with possible other pumping elements, the intake valve of which is open at the same time as that of the pumping element being considered;        by the volume included between the outlet of the metering solenoid valve and the intake valves of the pumping elements,        by the discharge head of the low-pressure pump; and/or        by the pressure regulated by a possible pressure regulator positioned in parallel with the metering solenoid valve.        
With regard to the timing of the metering solenoid valve command with respect to the top dead centre of a given pumping element, fixing the duration of activation of the metering solenoid valve, the filling coefficient of the pumping element considered shall assume a larger value in the case where the opening of the solenoid valve takes place when the pumping element is at bottom dead centre, which corresponds to maximum depression being “seen” by the same solenoid valve. In this case, the instantaneous flow of fuel supplied by the metering solenoid valve shall be the maximum, as it is proportional to the pressure difference between the inlet and outlet of the same solenoid valve, whereby the volume of fuel introduced shall be the maximum.
On the contrary, in the case of a pump with multiple pumping elements, the filling coefficient shall be a minimum if, at the moment the metering solenoid valve opens, all of the intake valves are closed (for example, also due to incorrect setting of the respective springs), whereby there will be no depression to aid the flow rate through the metering solenoid valve. The overall, or global, filling coefficient of the pump is a maximum if one or more of the intake valves of the other pumping elements are simultaneously open when the above-described conditions occur, whereby the depression “seen” in output from the metering valve is the maximum.
Since the control unit receives synchronization or timing signals from a phonic wheel carried by the engine drive shaft to generate the digital synchronization signals, these always have errors, albeit minimal, with respect to those supplied by the physical position of the engine drive shaft. This synchronization error can also derive from rounding errors in the pump cycle division calculation, especially in the case of a number of pumping elements that generate a periodic number as a quotient.
In these cases, the error generates slow slippage or scrolling, forwards or backwards, of the signals of the control unit with respect to the pump cycles. Therefore, whatever timing and synchronization is chosen for activating the metering solenoid valve during the delivery of the pumping elements, after a while, these deliveries will have faulty timing, generating ample pressure oscillations in the common rail having a relatively long period.
In particular, the more accurate the reading taken with the phonic wheel and the more precise the algorithm for calculating the frequency of operating the metering solenoid valve itself, the slower will be this slippage of the control signal for activating the metering solenoid valve with respect to the top dead centre of the respective pumping element taken as reference, and consequently, the longer will be the period of induced pressure oscillation.